By Julie Steenhuysen

CHICAGO, Sept 2 (Reuters) - With careful monitoring, even children who have had allergic reactions to a vaccine can still be vaccinated, a U.S. team of experts said on Tuesday.

The team developed a step-by-step guide to help pediatricians quickly identify children with allergic reactions to vaccines and safely immunize them.

"We cannot reiterate enough that the vaccines used today are extremely safe, but in a handful of children certain vaccine ingredients can trigger serious allergic reactions," said Dr. Robert Wood of Johns Hopkins Children's Center in Baltimore, whose research appears in the journal Pediatrics.

Many of these children can still be vaccinated for other diseases, he said.

The research comes amid recent outbreaks of measles, mumps and whooping cough in the United States, all of which can be prevented by vaccines.

Wood said that while very rare, occurring in the United States at a rate of about one to two per 1 million vaccinations, many pediatricians are likely to encounter allergic reactions simply because of the large volume of vaccines given, and they need to know how to proceed.

"There is very little in the way of current recommendations, which is one of the reasons we felt this was an important project to take on," Wood said in a telephone interview.

He said a true vaccine reaction will come on quickly, with 99 percent occurring within the first two hours of the shot.

The most common signs of a reaction are hives, swelling, nasal congestion, difficulty breathing and occasionally vomiting.

Children who have already had an allergic reaction or are at high risk should be tested by an allergist for reactions to the more common vaccine ingredients that trigger an allergic response. These include egg proteins, which are present in flu vaccines, or gelatin, which is used in several vaccines.

Wood said in many cases there are alternative vaccines that do not include these allergens.

"If a child had a reaction to the DPT (Diphtheria Tetanus Pertussis) vaccine and was found to have an allergy to gelatin, it would be fairly simple to give them an an alternative product," Wood said.

"If it were someone with a very severe gelatin reaction to the MMR (Measles Mumps Rubella) or varicella (Chickenpox) vaccines, you would have a more difficult dilemma about whether you would vaccine them or not," he said.

Another study in the same journal found the chickenpox vaccine caused a 90 percent drop in incidence of the disease, while a third recommended parents get vaccinated against whooping cough to protect their newborns.

(Editing by Will Dunham and David Wiessler)

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